The invention relates to a multilayer capacitor comprising alternate layers of a dielectric oxidic ceramic material and a palladium-containing electrode material, and having metallic end contacts which electrically conductively contact a prescribed part of the layers of electrode material, the metallic end contacts containing silver.
The invention also relates to such a multilayer capacitor, in which the metallic end contacts electrically conductively contact the layers of electrode material via an intermediate layer of a conductive metal oxide.
The invention also relates to a method of manufacturing such a multilayer capacitor.
In the U.S. Pat. No. 4,604,676 such a multilayer capacitor is described, in which the electrode material consists of a mixture of silver and palladium and use is made of an intermediate layer. The intermediate layer is a homogeneous thin layer having a thickness which must not exceed 2 .mu.m, the layer being applied by means of a thin-film deposition method such as sputtering, vapour deposition or chemical deposition from the vapor phase. Subsequently, for example, layers of Ni-Cr, Ni and/or Ag are applied by means of vapor deposition and sputtering so as to obtain solderable end contacts.
The intermediate metal oxide layer is provided to avoid a reduction of the ceramic material. Said reduction may occur as a result of diffusion of oxygen from the ceramic material to the metallic end contacts at a raised temperature, and it causes the insulation resistance of the ceramic material to be substantially reduced.
In an alternative method of manufacturing metallic end contacts, a mixture of metal, such as silver and, for example, palladium or other noble metals, and glass in the form of a paste is used. The paste can readily be applied by immersing and it can be fired together with the internal electrode layers of the multilayer capacitor. The metallic end contacts thus obtained contain a quantity of glass particles. However, in this method a number of problems are encountered, as will be explained hereinafter.
In the manufacture of multilayer capacitors, which are generally manufactured in large numbers, cracks are formed in a number of products and delamination occurs in the ceramic material near the end contacts. If these cracks reach the active part of the capacitor they may induce increased leakage currents, silver migration under the influence of an electric field and short-circuits between the inner electrodes. Moreover, cavities are then formed near the end contact and the bonding of the end contact material to the ceramic material is insufficient.
In experiments leading to the present invention it has been found that silver diffusion from the end contacts to the inner electrodes takes place predominantly during firing. The inner electrodes are made in the usual manner, preferably, of palladium which has a large affinity for silver. The volume and thickness of the inner electrodes increases locally which may lead to delamination and cracking. Outward migration of palladium caused by diffusion or increased pressure on the inner electrodes also takes place, albeit to a lesser extent. Owing to this, the material of the end contacts may become locally detached from the ceramic material and cavities may be formed. The formation of cavities may be counteracted by using a silver paste having a high glass-particle content, but then the problem arises that it is difficult to apply solderable layers onto such end contacts. Moreover, silver diffusion continues.